1. Field of the Invention
The present invention relates to a sapphire substrate for a nitride semiconductor light emitting device and a method for manufacturing the same, and a nitride semiconductor light emitting device incorporating the sapphire substrate.
2. Description of the Related Art
A light emitting diode (LED) made of a nitride semiconductor is usually constituted by sequentially laminating an n-type semiconductor layer, an active layer and a p-type semiconductor layer on a sapphire substrate. In this light emitting diode, emitted light is extracted from the side opposite the sapphire substrate or the side of the sapphire substrate, while light emitted from the active layer is also radiated in a direction opposite the light emission side. Therefore, it becomes necessary to improve external quantum efficiency by enabling light radiated in a direction opposite the light emission side to be extracted effectively from the light emission side.
For example, JP-A-2008-177528 discloses that external quantum efficiency is improved by arranging a plurality of truncated triangular pyramid-shaped projections on a sapphire substrate. JP-A-2008-177528 also describes that generation of voids and deterioration of crystallinity can be lessened by enabling crystal growth on a surface, on which truncated triangular pyramid-shaped projections are formed, through the projections.
However, the present inventors have intensively studied and found that crystallinity of a nitride semiconductor grown on a surface with truncated triangular pyramid-shaped dimples is not necessarily satisfactory.
With the increase of the output of a light emitting diode, problems due to crystal defects, which have not arisen in prior light emitting diodes, have recently arisen.